Granular forms of metals and alloys are desirable for use in many applications, notably where the metal or alloy is employed as feed stock to a melting process. For such a process, the attractions of using a granular feed as opposed to more conventional billet stock for example include the relative ease with which granulates can be melted in and uniformly distributed throughout a molten bath, as well as the potential for handling the granulates automatically and accurately metering the desired amount thereof.
Various techniques are known for producing metals and alloys in powder form. Such "atomization" techniques involve causing one or more atomizing streams of inert gas or water to impinge upon a stream of the molten metal to be atomized. Apart from the cost of such atomization processes, the resulting small particle size of the products inhibits their usefulness in many applications where, for example, dusting problems might be created. In such applications, it might be desirable to employ a particulate feed which is much coarser than the above mentioned powders, e.g., one consisting essentially of particles the diameter of which is greater than 2 mm or so, and preferably of the order of 25 mm or more. It is with the production of such particulate materials, rather than with powders, that the present invention is concerned, and the term "granulate" is used herein to denote such coarse particulate material.
Granulates have been produced for some time by the method commonly referred to as "shotting", wherein molten metal is discharged as a stream into a pool of water. While the technique is perhaps most closely associated with the production of lead shot, it has also been applied to metals of higher melting point than lead such as iron and steel. A recent process for the production of steel shot is described in U.S. Pat. No. 3,888,956, issued to N. J. Klint, in which a steel melt is poured as a vertical stream onto a horizontal flat surface of refractory material which causes the stream to be fragmented into droplets which then fall into a bath of cooling liquid. Drawbacks of this technique include frequent maintenance of the refractory material used as a disintegration surface, and the careful control needed to ensure that the stream of liquid metal to be disintegrated is approximately normal to the refractory surface at the point of impingement so that the metal stream is completely broken up. Apart from these drawbacks, the Klint patent does not provide an entirely satisfactory solution to the problem of producing nickel or cobalt shot which is suitable for remelting application.
When attempts are made to produce granulates of nickel or cobalt by the process described in the Klint patent, or by more conventional prior art shotting processes, two specific problems are encountered, namely, the tendency for the product to be in the form of smooth round granules and for these granules to possess undesirably high porosity. The sphericity of the granules is generally undesirable where they are intended for foundry use since they are unsuitable for handling by means of common conveyor belts and can pose safety hazards when industrial spills occur. Porosity of the granules is a more severe problem in that when granules of low density, i.e., having entrapped gases therein, are introduced into a molten bath, the sudden expansion of the entrapped gases leads to the phenomenon referred to as "thermal popping" whereby the added granules as well as some hot metal from the bath are made to spray out of the bath onto surrounding areas. The flying metal particles not only constitute a safety hazard, but also result in metal losses which may be substantial.